TURBINE CHARACTERISTICS	167
must run at that constant speed regardless of fluctuations in the
water. If trie speed is constant, it is seen tha-t <t>\/h = constant.
Thus if the head decreases, the value of <£ must increase. In
Fig. 109 may be seen the performance of a turbine at a constant
speed under different heads. By making a different choice of
the speed for the normal head, the results under all other con-
ditions will be altered, and careful study must be made of all
the.variables to decide what is best.
115. Specific Speed an Index of Type.—Both the elements of
speed and capacity are involved in the specific speed. It was
stated in Art. 38 that both speed and capacity were merely
relative terms; that is, a high-speed turbine is not necessarily
one which runs at a high r.p.m., but one whose speed is high
compared with other turbines of the same power under the same
head. In like manner a high-capacity turbine is not necessarily
one of great power but merely one whose power is high compared
with others at the same speed under the same head. Since
N_8 = —r^f^ ft *^s evident that a low-speed, low-capacity turbine
will be indicated by a low value of N_s and a high-speed high-ca-
pacity turbine by a high value of N_s. As stated in Art. 108,
values of N_8 for the tangential water wheel may run up as high
as 5 or 6, for the reaction turbine they range from 10 to 130.
Values in the neighborhood of 20 indicate a runner such as Type
I in Fig. 34, while values in the neighborhood of 80 indicate
Type IV. Thus when the speed and horsepower of any turbine
under a given head are specified the type of turbine necessary
is fixed.
Other things being equal, it is seen that a high head means a
comparatively low value of N_B while a low head means a high
value. Aside from any structural features it is apparent that a
high head calls for a tangential water wheel or a low-speed re-
action turbine, while a low head demands a high-speed reaction
turbine. However, the head alone does not determine the value
of N». So far as the r.p.m. is- concerned there may be consider-
able variation, yet neither a very low nor a very high r.p.m.
is desirable and for the present purpose we may suppose that it
is restricted within narrow limits. The value of N* will thus
be affected by the power of the turbine as well as the head. If
the head is high the value of N_s may still be high enough to
require a reaction turbine. Or if the head is very low and the